This section is from the book "A Treatise On Therapeutics, And Pharmacology Or Materia Medica Vol1", by George B. Wood. Also available from Amazon: Part 1 and Part 2.
About half a century ago, it was a prevalent belief that medicines operated on parts distant from the seat of their application by means of sympathy; in other words, by the transmission of their local impression through afferent nerves to the nervous centres in the encephalon, spinal marrow, or ganglia, and its radiation thence to the part or parts affected. In favour of this opinion, was, in the first place, the want of proof, in relation to the great majority of medicines, that they ever penetrated into the system beyond the original surface of contact; and, secondly, the great rapidity with which some of them produced their effects, which was supposed entirely to preclude the possibility of their actual conveyance to the seat of these effects. But, since it has been discovered that almost all medicines are capable of being absorbed into the circulation, and that perhaps in no instance are their effects felt in a time shorter than that required for their conveyance with the blood to the part acted on, the explanation of their operation by sympathy, or through the medium of the nerves, has become unnecessary, and, in reference to most medicines, is, I think, quite untenable.
It is true that many medicines produce local impressions, which are followed by effects in parts more or less distant, that cannot be explained upon the supposition of the transfer of the medicines themselves, and must, therefore, be brought about through nervous agency. Thus, tobacco and ammonia, applied to the mucous membrane of the nostrils, occasion sneezing, undoubtedly through the intervention of the nerves and the nervous centres; and in the same way we must account for such effects as the sweat and feeling of coldness in the forehead, sometimes following the application of common salt to the tongue, and the flow of tears and saliva resulting from horseradish in the mouth. But it will be observed that these are secondary effects, dependent on mere irritation of the part first impressed, and which might equally result from any other cause capable of producing an equal amount of irritation. They are not the primary or characteristic action of the medicines, which alone we are now considering. To fulfil the conditions of the theory of nervous transmission, it is necessary that the first impression on the surface of application should be conveyed unchanged, or merely modified; or at least that the remote effect should be of a characteristic nature, and not the same as that which any irritant might produce.
Yet it cannot be denied that there are facts tending to show that, in some instances, medicines do propagate their primary and peculiar influence through the nerves. Thus, certain fetid substances, which act efficiently as antispasmodics when introduced into the stomach or rectum, seem, by their mere impression on the olfactory nerves, to give rise to the same effects; and tobacco and lobelia occasion, by their presence in the fauces, nauseating impressions very similar to those produced by them when swallowed. I think every practitioner in the habit of administering opiates by enema, for the relief of nephritic and uterine pains, strangury, etc., must have witnessed a greater and more speedy effect from them thus given, than when taken into the stomach. This can be accounted for only by the transmission of their anodyne influence through the nervous centres; as, if absorbed into the circulation, they would reach the seat of action no sooner than if absorbed from the stomach, and, indeed, less speedily and efficiently; as medicines administered by the rectum are well known to affect the system less readily than when swallowed. It is, therefore, I Think, premature to reject altogether this mode of medicinal operation; and, indeed, it is not impossible that some substances may operate in both ways, giving rise to an impression through nervous transmission, which may be afterwards strengthened, and perhaps modified, by the immediate action of the medicine through the circulation.